1. Field of the Invention
The present invention relates to a technique for controlling power used for charging/discharging a secondary battery, such as a nickel hydrogen battery, a lithium ion battery, or the like mounted in an electric vehicle, a hybrid vehicle, or the like, as a power source for a motor or a drive source for various loads.
2. Related Art
On the occasion of usage of a secondary battery, such as a nickel hydrogen secondary battery, a lithium secondary battery, or the like, excessive discharge or excessive charge must be avoided. If a secondary battery is brought into a noticeably, excessively discharged state or a noticeably, excessively charged state, a gas is evolved by means of electrolysis of an electrolytic solution in a battery, which affects the life of the secondary battery.
Accordingly, in the field of an electric vehicle or a hybrid vehicle equipped with a secondary battery serving as a power source for a motor or a drive source for various loads, there has hitherto been proposed a technique in which a battery electronic control unit (hereinafter called a “battery ECU”) determines an allowable discharging power level and an allowable charging power level (hereinafter the allowable discharging power level and the allowable charging power level are collectively called “allowable power level”) which can be allowed by the secondary battery, in accordance with a charged state of the secondary battery (hereinafter called an “SOC” (State of Charge)), the temperature of the battery, and the like. An attempt is made to prevent excessive discharge or charge of a secondary battery; to prevent infliction of damage to the secondary battery or deterioration thereof; and to achieve longer life of the secondary battery, by means of a vehicle control unit (hereinafter called a “vehicle ECU”) controlling charging/discharging of the secondary battery in such a way that the allowable power level determined by the battery ECU is taken as a target level and is not surpassed.
Japanese Patent Laid-Open Publication No. 9-74605 describes a regenerative brake controller mounted in an electric vehicle equipped with a motor and comprising: means for determining an upper-limit charging power level from a state of a secondary battery; means for controlling an upper-limit control target value for regenerative brake from the upper-limit charging power level and the number of rotations of a motor; and means for diminishing the control target value for regenerative brake when the voltage of the battery has surpassed a predetermined allowable value.
Incidentally, in the case of a secondary battery which is mounted in an electric vehicle, a hybrid vehicle, or the like, and iterates charging/discharging operations within a short period of time, power does not become stable, and temporal variations are great, as shown in FIG. 11. Therefore, even when power has surpassed once a predetermined allowable value, power does not always surpass the allowable value continually. Shortly, even when power has surpassed a predetermined allowable value, excessive discharge or charge of the secondary battery does not always lead to infliction of damage to the secondary battery or deterioration thereof. Consequently, as in the case of the regenerative brake controller described in Japanese Patent Laid-Open Publication No. 9-74605, when the control target value for regenerative braking power decreases the voltage of the battery has surpassed the predetermined allowable value once, there may arise a case where the control target value is reduced more than necessary and charge of the secondary battery is restricted more than necessary.